A keyboard and particularly key buttons or keys of the keyboard are typically backlit so that the user can see the keys and particularly symbols or legends on top of the keys in dark conditions, too. It is known that portable electronic devices comprising a keyboard are provided with a light guide for guiding light from a light source to the function keys, or in short the keys. The light guide which is usually formed of light-transmitting rigid material is placed between a printed wired board (PWB) and a keyboard comprising keys to transfer the light from a light source, e.g. a plurality of light emitting diodes (LED), to illuminate the keys.
The light guide has a plurality of apertures, or holes, for permitting the conductor pads on the bottom of each key or a key dome placed between the key and the underlying PWB, to contact respectively positioned contacts on the PWB by short-circuiting a pair of electrical contacts on the PWB, when pressing a key. The plurality of LEDs used to light the keys, displays and the like are mounted, e.g. surface-mounted, on the upper side of the PWB to the proximity of the keys. When the rigid light guide layer is used on top of the PWB the plurality of LEDs may also be partly embedded directly into the light-transmitting material to the proximity of the keys so that the electrical contacts on the LED are brought into contact with the corresponding conductive pads on the PWB. The plurality of LEDs may be positioned to provide desired backlighting in different patterns. The LED may be disposed directly under each key, adjacent to each key, between two adjacent keys, at the interstice between four keys or other design choice depending on the illumination brightness desired. Typically vertical-emitting or side-emitting LEDs are used as light sources. The light transmitted by a plurality of LEDs is diffused into the translucent keypad and dispersed in a manner that provides backlighting for each of the keys of the keyboard.
Referring to figure 1a there is depicted a front cover 1, a keyboard 10 and a key top 5 of a mobile phone according to prior art. The cover 1 includes openings for the keyboard assembly in the locations of key tops 5. Also referring to figure 1a there is shown a cutting line A′-A′ along which a cross-section diagram is constituted depicted in figure 1b. 
As shown in FIG. 1b, prior art keyboards 10 typically comprise an elastic member 3, so called silicone base, which is formed so that ridges 17 on the upper surface of the elastic member 3 define the edges of the keys. Respectively, projections 19 on the lower surface of the elastic member 3 are located under each key. The rest part of the elastic member serves as a supporting frame between the keys so that under each ridge 17 of the elastic member 3 there exists a cavity for a dome 11. The plastic dome sheet 11a is typically provided with metal domes 11 on the side not attached to the light guide 7. The dome sheet 11a is also provided with the plurality of holes (not shown) through which the light reaches the light in-coupling surfaces on the light guide. The lower surface of the elastic member 3 exluding the cavity area is supported against the rigid light guide layer 7 underneath. The keyboard 10 has a plurality of keys and key tops 5 are attached on top of the keys so that each key top 5 extends through openings provided in a front cover 1. The individual keys with key tops 5 are typically separately depressed to bring the bottom of the key into contact with electrical contacts provided on the PWB 9 under the keyboard 10. Electrical components 4 are typically disposed on one side of the PWB 9 and electrical contacts or contact pads on other side of the PWB 9.
The openings of the front cover 1 are arranged so that they support each key top 5 extending through the opening. A lower perimeter of each opening is provided with an inwardly extending indent 13 and respectively, a lower perimeter of each key top 5 is provided with an outwardly extending lip 15 so that the indent 13 and the lip 15 are engaged along each other when pressed together. The front cover 1 provides a compressive force against the elastic keyboard 10 when assembled with the PWB 9, dome sheet 11a, rigid light guide 7 and a back cover 8 ensuring electrical functionality of the portable electronic device. This means that the keyboard assembly 10 according to prior art typically provides the use of two-part cover, i.e. the front cover 1 and the back cover 8 as well as means for fixing the covers together (not shown), to be installed into the mobile phone.
In prior art the keyboard assemblies 10 are provided with a number of LEDs 2 to get the required illumination in dark conditions for the keyboards. LEDs 2 are surface-mounted on the upper side of the PWB 9 to the proximity of the keys and they may be partly embedded into cavities 2a machined to the light guide 7. In combination with the light guide 7 typically at least six to eigth LEDs 2 are needed to illuminate the keyboard assembly 10 of the mobile phone. LEDs 2 are rather large-sized components compared to other electrical components used in portable electronic devices. This means that the light guide layer 7 will be relatively thick in the keyboard assemblies 10 according to prior art. In addition, LEDs 2 are rather power consuming components which means that the larger the number of LEDS 2, the larger the power consumption. In addition, prior art keyboard assemblies 10 are arranged so that each key top 5 extends through the opening of the front cover 1, the key top 5 being the uttermost part of the device, which means that key tops 5 can be easily pulled out of the opening or damaged against mechanical stress, if any additional protection frame around the key tops 5 is not used. This situation may occur for example when user holds the mobile phone in his/her pocket unprotected. In addition, prior art keyboard assemblies provided with openings in the cover part 1 to support the whole combination of keyboard assembly 10 as described above, will limit the freedom to design keyboard assemblies independently, because the layout design of the cover part is bound up with the layout design of the keyboard.
The problems set forth above are overcome by providing a keyboard assembly for portable electronic devices having a lesser number of LEDs to achieve required illumination brightness resulting in thinner light guide structures, smaller power consumption and savings in material and production costs. In addition, protection and supporting features combined to the keyboard assembly according to the invention provide small-sized and less weighing keypad structures and consequently smaller keyboards for portable electronic devices. In addition, integrated protection features improve the mechanical robustness of the keyboard assemblies and consequently the robustness of the portable electronic devices having the keyboard assembly according to the invention. In addition, the keyboard assembly according to the invention gives more freedom to design a layout of the keyboard, as well as the portable electronic device having the keyboard assembly according to the invention.